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From an Enediyne Core Biosynthetic Hypothesis to the Hexadehydro-Diels–Alder Reaction

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From an Enediyne Core Biosynthetic Hypothesis to the Hexadehydro-Diels–Alder Reaction Spontaneity to Serendipity: From an Enediyne Core Biosynthetic Hypothesis to the Hexadehydro-Diels–Alder Reaction A DISSERTATION SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Brian Patrick Woods IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Thomas R. Hoye, Adviser August 2014 © Brian Patrick Woods 2014 i Acknowledgements While my name is on the first page of this thesis, countless family, friends, and colleagues contributed their knowledge, support, and love to help make this document a reality. First and foremost, I need to thank my family. I owe everything to my parents, Steve and Diane, who were my first teachers and have been and continue to be inspiring examples of how a life of constant learning and exploration never gets old. They instilled in me their value in the benefits of education and the doors it opens for those willing to knock. For both my siblings and I, they always encouraged us to pursue our dreams and desires—from CSI investigator, to Broadway actor, to NASA scientist—and provided us with the guidance and support to reach them. To my sister and brother, Shannon and Tyler, I thank them for their innate ability to keep me grounded and their unwavering confidence in me. To each of them, along with their partners Braxton and Esther, and my nephew Harrison, I thank them for being a constant reminder that there’s nothing more important than family. For their continual love I would also like to thank my extended family of aunts, uncles, and cousins—and more specifically my grandparents Patrick and Virginia Woods. Additionally, for their friendship throughout my time at Minnesota and help in maintaining the requisite levity and sanity in my life, I thank Antonio Campos, Ryan Knutson, Jeff Vervacke, and especially my roommates Jeremy Bedard and Drew Thompson. From a more professional standpoint, I first need to give the utmost thanks to my adviser, Thomas R. Hoye. Tom has been a phenomenal adviser, going above and beyond what I expected when I arrived at graduate school. He has a passion for chemistry and particularly teaching that is contagious. Both a brilliant scientist and incredibly effective mentor, Tom has an appreciation for the beauty and intricacies of organic chemistry that shapes the tone of his group. His commitment and loyalty to his students is especially commendable. From every stage of graduate school he has always readily offered useful guidance and insight, related to course schedules in the first year to career advice in the fifth year. I thank him for taking a chance on an eager new student from a small ii undergraduate college five years ago and allowing me to join his research group. I would not be anywhere near the teacher, researcher, writer, or overall scientist I am today without his mentorship. I also need to thank the rest of the outstanding faculty at the University of Minnesota who have had a hand in shaping my growth as a teacher and scientist. In particular, I thank Jane Wissinger. Jane guided me through my first year as a Teaching Assistant and helped show me the gratification and fulfillment that comes from effectively taking a group of students through a semester of Organic Chemistry. Working closely with Jane for three years as the Head Organic TA she became like a second adviser to me. I learned innumerable leadership, motivational, and educational skills from her for which I will always be appreciative. In the same context I would like to thank Michael Wentzel for allowing me to attend and guest-lecture in his Organic Chemistry class at Augsburg College. Mike was my graduate student contact at Minnesota for my first visit, and has been an inspirational mentor and friend ever since. Next, I would like to thank my committee members Steve Kass, Christopher Douglas, and Carston Wagner. They have been patient, flexible and supportive as I progressed through the written and oral preliminary exam and on to the final thesis submission and oral defense. For their generosity and help with the use of their DSC instrument, I am grateful to Marc Hillmyer and his group. To my professors Christopher Cramer, Andrew Harned, Valerie Pierre, and again Tom, Chris Douglas, and Steve Kass, I give thanks for their valuable coursework. I would also like to thank Letitia Yao for her admirable work maintaining the essential NMR facilities, and Laura J. Clouston, Victor G. Young, and the X-Ray Crystallographic Laboratory for their determination of the crystallographic data presented in this Thesis. Throughout my studies at Minnesota, I have benefited immensely from being surrounded by an exceptionally talented group of colleagues. The majority of my work was done in collaboration with Beeru Baire, Dawen Niu, and Patrick Willoughby, or “Team Benzyne” as we came to call ourselves. Being involved in the early stages of the HDDA project with these three creative, driven, and genuinely good-natured colleagues iii made coming to work every day exciting, rewarding, and enjoyable. The project would not have been nearly as productive without such a cordial and spirited collaboration. Specifically, I am forever indebted to my senior graduate students Patrick and Dawen for the prolific advice and guidance I gained on a daily basis simply from observing how they handle themselves as scientists. For early encouragement from my time as an overwhelmed and naïve new graduate student, I thank Mandy Bialke and Susie Emond for welcoming me to my lab in Smith 413. They were the first to introduce me to the ways of the Hoye lab and, more importantly, the mechanics of the MPLC setup. I thank my current Smith 413 tenants Vedamayee Pogula, Xiao Xiao, Moriana Haj, Quang Luu Nguyen, and Andrew Mullins for maintaining the friendly atmosphere that Mandy and Susie established. Matthew Jansma was my first mentor from a research standpoint, and I benefited immensely from witnessing his work ethic, preparation, and bench skills on a day-to-day basis, not to mention his role in teaching me everything I needed to know to operate and maintain the group GC-MS. I thank Joshua Marell and Xiangyun Lei from the group of Chris Cramer for their extensive computational analysis in what turned into an enjoyable and fruitful collaboration. For their friendship and helpful subgroup meetings I thank former group members Adam Wohl, Susan Brown, Julian Lo, and Eric Buck. The current group members have also contributed greatly to the state of the HDDA project with vigor and enthusiasm; the project has continued to thrive thanks to the brilliant work of Junhua Chen, Tao Wang, and Sean Ross. I have had the opportunity to work with two terrific undergraduates during my time as a graduate student, Moriana and Quang, who I thank for their patience, assistance, and outstanding work. I need to give particular thanks to my colleague Andrew Michel for the monumental task of maintaining (for the most part) the group LC-MS, for his friendship, and for constructive and essential pre-group meetings. I am also very thankful to Moriana, Sean, Junhua, and Andrew for critically reviewing portions of this thesis. iv To my Parents My lifelong teachers and role models v Abstract Enediyne containing natural products have promising potential as cancer therapeutics due to their unique molecular architecture. The (Z)-1,5-diyn-3-ene subunit in the enediyne core can undergo cycloaromatization to yield a diradical capable of scission of the DNA double helix. While the biological mechanism of action is well established, almost nothing is known about the biosynthesis of the enediyne core. Specifically, researchers have been unable to identify a cyclase enzyme capable of ring- closing acyclic precursors. In the case of 9-membered enediynes, we propose that the bicyclic enediyne core is formed biosynthetically via spontaneous (i.e. non-enzymatic) cyclization from an acyclic precursor. In the course of examining this hypothesis, we serendipitously encountered a [4+2] cyclization between a diyne and an alkyne. The product of such a cycloaddition is one of the oldest and most interesting reactive intermediates in organic chemistry, o-benzyne. This process, which we have termed a hexadehydro-Diels–Alder (HDDA) reaction, has remained almost entirely unexploited until now. The strategy unites an entirely atom-economical, thermal generation of arynes with their in situ elaboration into a diverse set of polysubstituted benzenoids. HDDA precursor triynes cycloisomerize in a very exergonic fashion to produce complex benzyne intermediates, which are trapped with a variety of inter- and intra-molecular functionalities in an efficient and selective manner. The byproduct-free environment in which the benzynes are generated allows for new trapping reactions to be discovered and for mechanistic pathways to be interrogated and elucidated. vi Table of Contents Acknowledgements i Dedication iv Abstract v Table of Contents vi List of Figures ix List of Tables xii List of Abbreviations xiii Part I: Investigation of a Spontaneous Cyclization in the Biosynthesis of the 9-Membered Enediyne Natural Products Chapter I: Background and Biosynthetic Hypothesis 2 1.1 Introduction to the Enediyne Natural Products ..................................................................2 1.2 Biosynthetic Hypothesis for 9-Membered Enediyne Core ...............................................4 1.3 Previous Synthetic Studies Relevant to 9-Membered Enediynes ......................................5 1.3.1 Sondheimer Chemistry 5 1.3.2 Enediyne Core
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